How many grapes to buy?

I'm going to pick up about 100lbs of chancellor grapes this weekend. I'm fairly new to wine making, done several kits and a couple small batches with my own grapes. Anyway the problem I forsee is ending up with 5 gallons of wine in a six gallon carboy. I've never really come across this issue before. Do I just have to get smaller carboys to keep airspace to a minimum? I have read that you can fill that space with a different gas to keep oxygen out but I have no idea if that's true. I could try to reserve another 20lbs of grapes as well just to be sure I'll get. 6 gallons.

You should have enough for 6 gallons plus a little. I would get a half gallon jug for what does not fit in the 6 gallon carboy.

avg yield is 12-15 lbs of grapes per gallon. 100lbs could yield 7 gallons on high side.

All above are right.... But to answer your question. You may want to invest in a few pounds of glass marbles for the future.....

I would suggest that if you are buying 100 pounds of grapes and you have a 6 gallon carboy, you will need either a 1 gallon or a half gallon jug for the overage you will have. I would be very surprised if you do not end up with more than 7 gallons of wine assuming you crush and press well. Good luck.